The organic electroluminescent device is a self-luminous device and has been actively studied for their brighter, superior visibility and the ability to display clearer images in comparison with liquid crystal devices.
In 1987, C. W. Tang and colleagues at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic electroluminescent device with organic materials. These researchers laminated an electron-transporting phosphor which is tris(8-hydroxyquinoline)aluminum (hereinafter referred to as Alq3) and a hole-transporting aromatic amine compound, and injected both charges into a phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (refer to Patent Documents 1 and 2, for example).
To date, various improvements have been made for practical applications of the organic electroluminescent device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescence device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode successively formed on a substrate (refer to Non-Patent Document 1, for example).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and the use of phosphorescent materials has been examined (refer to Non-Patent Document 2, for example).
The light emitting layer can be also fabricated by doping a charge-transporting compound generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing lecture preprints, the selection of organic materials in an organic electroluminescent device greatly influences various device characteristics such as efficiency and durability.
In an organic electroluminescent device, charges injected from both electrodes recombine in a light emitting layer to cause emission. What is important here is how efficiently the hole and electron charges are transferred to the light emitting layer. The probability of hole-electron recombination can be improved by improving hole injectability and electron blocking performance of blocking injected electrons from the cathode, and high luminous efficiency can be obtained by confining excitons generated in the light emitting layer. The role of a hole transport material is therefore important, and there is a need for a hole transport material that has high hole injectability, high hole mobility, high electron blocking performance, and high durability to electrons.
Heat resistance and amorphousness of the materials are also important with respect to a lifetime of the device. The materials with low heat resistance cause thermal decomposition even at a low temperature by heat generated during the drive of the device, which leads to the deterioration of the materials. The materials with low amorphousness cause crystallization of a thin film even in a short time and lead to the deterioration of the device. The materials in use are therefore required to have characteristics of high heat resistance and satisfactory amorphousness.
N,N′-diphenyl-N,N′-di(a-naphthyl)benzidine (hereinafter referred to as NPD) and various aromatic amine derivatives are known as the hole transport materials used for the organic electroluminescent device (refer to Patent Documents 1 and 2, for example). Although NPD has desirable hole transportability, it has a low glass transition point (Tg) of 96° C. which is an index of heat resistance and therefore causes the degradation of device characteristics by crystallization under a high-temperature condition (refer to Non-Patent Document 3, for example). The aromatic amine derivatives described in the Patent Documents 1 and 2 include a compound known to have an excellent hole mobility of 10−3 cm2/Vs or higher. However, since the compound is insufficient in terms of electron blocking performance, some of the electrons pass through the light emitting layer, and improvements in luminous efficiency cannot be expected. For such a reason, a material with higher electron blocking performance, a more stable thin-film state and higher heat resistance is needed for higher efficiency.
Arylamine compounds of the following formulae having a substituted acridan structure (for example, Compounds A and B) are proposed as compounds improved in the characteristics such as heat resistance, hole injectability and electron blocking performance (refer to Patent Documents 3 and 4, for example).

However, while the devices using these compounds for the hole injection layer or the hole transport layer have been improved in heat resistance, luminous efficiency and the like, the improvements are still insufficient. Further, it cannot be said to have a sufficiently low driving voltage and sufficient current efficiency, and there is a problem also in amorphousness. Further improvements of a low driving voltage and luminous efficiency while increasing amorphousness are therefore needed.